The use of energy storage batteries in combination with a generator is known for automobiles, buses and other road and highway vehicles. Electric batteries have been used to store electric power to drive electric locomotives as, for example, disclosed by Manns in U.S. Pat. No. 1,377,087 which is incorporated herein by reference. Donnelly has disclosed the use of a battery-dominant hybrid locomotive which has a ratio of energy storage capacity to charging power in the range of 4 to 40 hours in U.S. Pat. No. 6,308,639 which is also incorporated herein by reference.
One of the principal objectives of hybrid locomotive design is to operate the locomotive in such a way as to maximize the lifetime of its energy storage unit. This is because the cost structure of an energy storage unit such as for example a battery pack or capacitor bank is primarily one of capital cost and secondarily of operating costs. It is known, for example, that operating a lead-acid battery pack in a preferred state-of-charge (“SOC”) range or with a preferred charging algorithm or with both tends to extend serviceable lifetime of cells in cyclical service towards that of float service.
Large energy storage battery systems are known, for example, from diesel submarines. In this application, a pack of large storage batteries are used to provide power principally when the submarine is operating underwater. These submarine battery packs are designed to provide high energy storage capacity for extended underwater operations during which the battery pack cannot be recharged. Battery pack cost and lifetime are generally not major concerns.
In the late 1990s, a large stationary battery system was installed at the island village of Metlakatla, Ak. The 1.4 MW-hr, 756 volt battery system was designed to stabilize the island's power grid providing instantaneous power into the grid when demand was high and absorbing excess power from the grid to allow its hydroelectric generating units to operate under steady-state conditions. Because the battery pack is required to randomly accept power as well as to deliver power on demand to the utility grid, it is continuously operated at between 70 and 90% state-of-charge. Equalization charges are conducted during maintenance periods scheduled only twice each year.
It has long been thought that to achieve optimum life and performance from a lead-acid battery, it is necessary to float the battery under rigid voltage conditions to overcome self-discharge reactions while minimizing overcharge and corrosion of the cell's positive grid. This has resulted in batteries being used primarily in a standby mode. As used in a hybrid locomotive or as a power grid storage and control system, the battery is rapidly and continuously cycled between discharge and charge over a preferred range of total charge.
It has been possible to assess aging and performance capabilities over time in this controlled cycling type of service by detailed monitoring. Data has been generated to demonstrate the long-term viability of cells in this type of use, performing functions such as load leveling, peak shaving and power quality enhancement. Detailed examination of the cells plates and separators have shown little wear indicating that controlled operation such as described above can result in battery lifetimes that can approach design lifetimes associated with float service.
However, there remains a need for a more comprehensive procedure suitable for designing large battery pack assemblies with long lifetimes for hybrid locomotives that satisfies a number of diverse requirements for locomotive performance, maintenance, safety and cost-effective operation.
A principal design objective for many applications is maximum energy storage capacity. When this objective is achieved, the power output of the battery pack is usually more than sufficient. In many applications, a principal design objective is low capital and operating cost. This usually means a lead-acid battery with some compromise in power or capacity. In applications such as hybrid locomotives used as yard or road switcher locomotives or commuter locomotives, maximum power out is a principal design objective. A further principal design objective is battery pack lifetime since this directly relates to the unit cost of power supplied indirectly through a battery system.
The design objectives of a large battery pack for a hybrid locomotive has a unique set of problems to achieve its principal design goals of high storage capacity, high power on demand, cyclical operation, long lifetime and a cost effective design for a large battery pack. These objectives must be met on a locomotive platform subject to shock and vibration as well as extreme changes in ambient temperature conditions. There therefore remains a need for a battery design for a hybrid locomotive that is capable of operation requiring a combination of high storage capacity, high power capability for rapid acceleration, long battery pack lifetime, and a low cost capital and replacement cost structure under diverse locomotive operating environments. These and other objectives are met by the design approach described in the present invention.